FLOOD04641

• County: Denver
• Community: All
• Stream Name: All
• Basin: South Platte
• Title: Proposed Hydrology for use in Hydraulic Analysis of FEMA Flood Studies and Restudies
• Date: 9/1/1989
• Prepared For: State of Colorado
• Prepared By: Resource Consultants
• Floodplain - Doc Type: Educational/Technical/Reference Information

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equation. The rainfall depths determined for all basin
applications are presented In Table 1.1.

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TABLE U

FAINFALL DEPTH FOR CRITICAL DESIGN STORM

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STORM RAINFALL DEPTH (inches)
DURATION {hoursl 10 YR 50 YR 100 YR
24 (General Colorado Springs! 3.10 3.90 4.40
El Paso County)
24 (West Colorado Springs) 3.20 4.00 4.50
1 (For deriving the 2-hr. storm) 1. 78 2.35 2.64
2 (Derived from the I-hr. storm) 2.06 2.72 3.05

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The SCB Type lIA dIstribution for 24-hour duration is an
intense rainfall occurrence whereby a majority of the
precipitatlon occurs betl<een the fifth and sixth hour of the
storm. Distribution for the 2-hour storm is also intense, with
a majority of the precipitation occurring between 20 and 35
minutes after initIatlon of the rainfall. Both distributions,
as presented in Figures 5-5a and 5-5b of the DCM, are
reprodwoed in Figure 1.2.

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There is a single curve that applies to the 10-, 50-, and 100-
year storm events under the 24-hour Type IIA distribution.
Separate curves for the 10- and 100-year events are required
for the 2-hour duratIon storm. No distribution is specified
for the 50-year, 2-hour duration storm. Based on engineering
judgment, It was decided to evaluate the 50-year event with the
same distribution as for the lOO'year, 2-hour duration storm.

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Anteced'mt moisture conditions (AMC) as defined by the SCS have
been prescribed for use with the tl<O storm distributions by the
DCM. An AMC II condition is specified for the 24-hour storm
and an AMC III condition is specified for the 2-hour storm.
AMC II reflects an average moisture condition in the soil
whereby some infiltration will occur before runoff starts. AMC
111 indicates saturated conditions with little potential for
infiltr.,tion.

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1.4

Recommended Storm Criteri-a

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Guidelines set forth under the DCM were relatively new at the
time of this study and little experience was available
concerning their applicability. In particular, the AMC III
condition imposed for the 2-hour storm had rarely been used in
the Colorado Springs area. After the initial hydrologic
analysis was complete for all fifteen basins being studied, an
attempt was made to determine if there were any significant
factors for selection of flows developed under the 24-hour AMC

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RE:~OURCE: CON~ULTt\NT~ INC

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